INS and obesity due to melanocortin 4 receptor deficiency: A physiological hallmark of obesity is insulin resistance.37 For decades, prevailing theory discounted a direct role for insulin in lactation.38,39 Beginning in 2009, in vitro studies challenged this theory, demonstrating a direct and essential role for insulin in lactation.40–43 In addition, research demonstrates that insulin stimulates milk protein and lipid biosynthesis in human lactation.44 The first author and collaborators have consistently reported associations between glucose intolerance and negative lactation outcomes in human studies.8,10,45